The power output demand on a power plant grid system varies greatly during the course of a day or week. During intermediate and high demand periods, typically between 7:00 a.m. and 11:00 p.m. on weekdays, the value of electric power is comparatively high. In contrast, during the low demand periods, typically on weekends and between 11:00 p.m. and 7:00 a.m. on weekdays, the value is relatively low. Thus, for the low demand periods, it would be highly advantageous to find an efficacious way to (1) store the mechanical, thermal, and/or electrical output of an individual power plant, or (2) store the electrical output produced by other power plants on the grid. The stored power could then be economically used during high demand periods.
One approach to storing power generated during low demand periods involves the operation of compressors during these periods. The compressors produce compressed air which possesses mechanical and thermal energy which can be stored. The compressed air from storage may be utilized by the power plant at a later time while the compressors are shut down. While this approach realizes certain benefits, there are still some deficiencies associated with it.
First, the capital cost and operating costs of compressors are high. Another issue relates to the practical requirement of cooling the compressed air before storage and then heating the compressed air after it is removed from storage. This heating is generally accomplished through recuperation and combustion of a carbonaceous fuel, which is expensive and results in the emission of pollutants. Prior art compressed air storage plants, even those with recuperators, do not utilize the exhaust thermal energy as efficiently as possible. The amount of carbonaceous fuel consumption, and hence emissions, can be reduced through a more efficient use of exhaust thermal energy generated in the power plant.
These problems associated with compressed air storage have precluded the use of compressed air storage in fuel processing power plants (i.e., power plants with a major fuel processing system, such as a coal gasification power plant). There are a number of problems associated with fuel processing power plants which could be solved through proper utilization of a compressed air energy storage facility. One problem associated with fuel processing power plants relates to the high capital cost associated with fuel processing equipment. It would be advantageous to eliminate the fuel processing equipment associated with providing power to the compressor during high demand periods. Another issue with fuel processing power plants relates to altering the power output during the course of a day to address high demand and low demand periods. It would be advantageous to operate such a power plant such that it approaches a steady state condition.